The present invention relates to a medical implant and method, and, more particularly, to an improved surgical implant and method for expanding the spinal canal to eliminate pressure on the spinal cord caused by an impinging vertebral bone.
Various medical conditions may result in a reduction of the area within the vertebrae available for the spinal cord. Spinal stenosis is one such condition involving the narrowing of the canal in the center of the spine through which the spinal cord and nerve roots run. Spinal stenosis may result when the ligaments of the spine thicken and calcify (harden from deposits of calcium salts), or when bones and joints enlarge, and osteophytes (bone spurs) form. A herniated (bulging) disk may also place pressure on the spinal cord or nerve root. Furthermore, diseased bone or tumors may result in an ingrowth into the spinal cord area. This decreases the space (neural foramen) available for nerve roots leaving the spinal cord.
Two surgical methods currently exist to create additional room in the spinal canal. The first is called a laminectomy, and involves removal of the lamina (roof) of one or more vertebrae. A limitation of the laminectomy procedure is that it involves removal of the supporting structures at the back of the vertebrae which align the spinal column. The result may be that a patient suffers some postural deformity. To prevent such postural problems, a graft may be installed between the ends of the removed bone to span the void and reinstate the necessary support. The second procedure is called a laminoplasty, in which the targeted vertebra is cut, spread apart and a graft is inserted to permanently enlarge the space. Unlike the laminectomy, typically no bone material is excised during the laminoplasty procedure. Two different laminoplasty procedures are currently used. The first is called the unilateral or xe2x80x9copen doorxe2x80x9d laminoplasty in which one side (lamina) of the vertebra is cut all the way through, while the other side is cut only half way to create a hinge. The vertebral element is then rotated about the hinge, and the graft is inserted into the opening, increasing the opening of the spinal canal. The second procedure is called the bilateral or xe2x80x9cFrench doorxe2x80x9d laminoplasty in which the midline of the vertebra (spinous process) is cut all the way through, and the lamina are cut half way through, creating two hinges. The vertebral element is then opened at the bisected spinous process, and a graft inserted into the opening, again increasing the opening of the spinal canal.
Various materials may be used for the grafts installed during laminoplasty procedures. U.S. Pat. No. 6,080,157 to Cathro et al. and U.S. Pat. No. 5,980,572 to Kim et al. disclose the use of titanium, ceramic and nylon inserts. Further, using allografts taken from long bones such as the femur, humerus, tibia and fibula, for spinal fusion procedures is known, as disclosed by U.S. Pat. No. 5,728,159 to Stroever et al. Allografts, as such bone grafts are called, are removed from a donor and processed using known techniques to preserve the allograft until implantation. Allografts have mechanical properties which are similar to the mechanical properties of vertebrae even after processing. The benefit of such property matching is that it prevents stress shielding that occurs with metallic implants. Allografts, unlike magnetic metals, are also compatible with magnetic resonance imaging (MRI) procedures, allowing more accurate ascertainment of fusion. Furthermore, allografts are naturally osteogenic providing excellent long term fusion with the patient""s own bone.
Several different spacer designs have been used in laminoplasty procedures to the present. For example, the Cathro patent discloses a metal, nylon or teflon spacer for use in a unilateral laminoplasty procedure. The Cathro spacer is a rectangular plate having shouldered edges which engage the ends of the cut lamina, and is held in place by a spring mechanism. The difficulty with the Cathro spacer is that its operation relies on the continued satisfactory operation of the installed spring. Further, the Cathro device provides little available area for the packing of fusion enhancing (i.e. osteogenic) material. The Kim patent discloses a spacer for use in a bilateral laminoplasty procedure. The Kim spacer consists of inner and outer trapezoidal segments joined together by a rectangular segment. The tapered surface of the inner trapezoidal segment is designed to conform to the inner surface of the split spinous process halves, while the taper of the outer segment is designed to assume the shape of the removed spinous process tip. The Kim spacer seats on the resulting flat surface of bone. Like the Cathro device, the Kim device provides little area in which to pack osteogenic material to facilitate bone-implant fusion. Neither the Cathro nor Kim device use allograft as a spacer material, which may result in reduced propensity for fusion and the possibility for stress shielding.
Accordingly, there is a need in the art to provide implants and methods for both laminectomy and unilateral and bilateral laminoplasty procedures, which provide excellent dimensional, strength and retention capability, which enhance fusion with the patient""s own bone, which are easy to select, fit and install and which provide excellent compatability with post-operative imaging (MRI).
The implants of present invention are provided for use in the spinal column. In one embodiment, the implants comprise an allograft fabricated from cancellous bone material and a member formed of non-allograft material having first and second bone engaging portions and an allograft engaging portion. The graft engaging portion may be configured to retain the allograft when the allograft contacts the graft engaging portion.
The graft engaging portion may comprise at least one raised tab. Further, the implant member may have a central region between the first and second bone engaging portions and the at least one raised tab angled inward toward the central region of the member. The allograft may have first and second ends, each comprising bone engaging portions, where at least one of the bone engaging portions is comprised of partially, substantially, or fully demineralized bone. At least one of the implant member bone engaging portions may comprise a suture attachment portion configured to allow a surgeon to secure the member bone connecting portions to the first and second bone segments.
In a different embodiment, an implant is provided for use in maintaining a desired distance between a first spinal bone cut end and a second spinal bone cut end, in which the implant comprises an allograft having a body and first and second ends, and a plate formed of a non-allograft material having an intermediate portion and first and second ends, where the intermediate portion has an allograft engaging portion configured to retain the allograft, and where the first and second ends of the plate have bone engaging portions which themselves have fastener receiving portions. The allograft engaging portion is configured to engage the allograft body and the bone engaging portions are configured to engage respective outer surfaces of first and second spinal bone cut ends. The allograft first and second ends are configured to contact the first and second cut bone ends. In a specific embodiment, the allograft engaging portion may comprise deformable fingers configured to engage the graft. In another specific embodiment, the allograft engaging portion may comprise a hollow portion, where the allograft has a shape complementary to the hollow portion, and where the hollow portion is configured to at least partially receive the allograft. In a further embodiment, the allograft first and second ends comprise bone engaging portions, at least one of which may comprise partially, substantially, or fully demineralized bone.
A method for providing a desired distance between first and second cut bone ends of the spine is also provided. This method comprising the steps of: cutting a vertebra to produce first and second cut bone ends; separating the bone ends to define a space therebetween; providing an allograft having a body and first and second ends; providing a plate formed of a non-allograft material having an intermediate portion and first and second ends, where the intermediate portion has an allograft engaging portion configured to retain the allograft, the first and second plate ends have bone engaging portions with fastener receiving portion, and where the allograft engaging portion is configured to engage the allograft body, the bone engaging portions are adapted to engage the first and second bone outer surfaces, and the allograft first and second ends are configured to contact the first and second cut bone ends, then engaging the allograft engaging portions of the plate with the allograft; engaging the bone engaging portions with respective cut bone ends; providing at least two bone fasteners; inserting at least one fastener into the fastener receiving portion of each bone engaging portion; and engaging the at least one bone fasteners with said cut bone end. In a further embodiment, the step of cutting a vertebra may comprise cutting all the way through one lamina. In a further embodiment, the adjacent lamina further may be cut half way through.